Colloidal PbSe Nanocrystals Research Articles

Lilac Amirav

Lilac Amirav

Near-Infrared Photodetectors Based on Hybrid Graphene-Colloidal PbSe Quantum Dots

AbstractPhotodetectors based on a hybrid structure of graphene sensitized with lead selenide (PbSe) colloidal quantum dots (QDs) effective in the near-infrared (NIR) region with high responsivity were investigated. Colloidal PbSe nanocrystals were synthesized via a hot injection method. The bandgap of the synthesized nanocrystals was determined to be 0.68 eV by measuring their optical absorbance spectrum. Photodetectors based on PbSe QDs were investigated to examine their functionality. These devices were characterized by measuring the current-voltage curves in the dark and light and the spectral response spectrum. A photodetector was fabricated using a multilayer mechanically exfoliated graphene on a Si/SiO2 substrate with a PbSe QDs layer on top. A responsivity and detectivity of 1265A/W and 3.4 *1010cm.Hz0.5/W respectively were calculated based on current-voltage measurements.

Infrared light-emitting diodes based on colloidal PbSe/PbS core/shell nanocrystals**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0401702), the National Natural Science Foundation of China (Grant Nos. 61674074 and 61405089), Development and Reform Commission of Shenzhen Project, China (Grant No. [2017]1395), Shenzhen Peacock Team Project, China (Grant No. KQTD2016030111203005), Shenzhen Key Laboratory for Advanced Quantum

Colloidal PbSe nanocrystals (NCs) have gained considerable attention due to their efficient carrier multiplication and emissions across near-infrared and short-wavelength infrared spectral ranges. However, the fast degradation of colloidal PbSe NCs in ambient conditions hampers their widespread applications in infrared optoelectronics. It is well-known that the inorganic thick-shell over core improves the stability of NCs. Here, we present the synthesis of PbSe/PbS core/shell NCs showing wide spectral tunability, in which the molar ratio of lead (Pb) and sulfur (S) precursors, and the concentration of sulfur and PbSe NCs in solvent have a significant effect on the efficient PbS shell growth. The infrared light-emitting diodes (IR-LEDs) fabricated with the PbSe/PbS core/shell NCs exhibit an external quantum efficiency (EQE) of 1.3 % at 1280 nm. The ligand exchange to optimize the distance between NCs and chloride treatment are important processes for achieving high performance on PbSe/PbS NC-LEDs. Our results provide evidence for the promising potential of PbSe/PbS NCs over the wide range of infrared optoelectronic applications.

PbSe quantum dot films with enhanced electron mobility employed in hybrid polymer/nanocrystal solar cells

We explore two strategies to improve the performance of hybrid solar cells fabricated using poly(3-hexylthiophene) (P3HT) and colloidal PbSe nanocrystals, which have reached a 1 sun power conversion efficiency of 2.9%.

Chalcogenidometallate Clusters as Surface Ligands for PbSe Nanocrystal Field-Effect Transistors

We introduce a method to process colloidal PbSe nanocrystals (NCs) into inorganic NC thin films using chalcogenidometallate (ChaM) clusters as surface ligands, resulting in electrically coupled NC solids. NCs are first immobilized on a substrate via a self-assembled monolayer followed by chemical treatment to exchange the insulating oleate ligands with ChaM clusters. Quantum confinement in the PbSe NCs is preserved as evidenced by persistent excitonic features in the absorption spectrum. PbSe NC–ChaM composites exhibit rectification (“off” states), saturation, n-type electrical behavior, and high electron mobilities of 1.28 and 0.475 cm2 V–1 s–1 for different composite compositions.

The exciton and phonon coupling in temperature-dependent photoluminescence of colloidal PbSe nanocrystals

The exciton and phonon coupling is an important factor to affect the photoluminescence of nanocrystals. The temperature- and size-dependent photoluminescence was employed in this paper to investigate the influence of exciton and phonon coupling in colloidal PbSe nanocrystals. The experimental results show that the photoluminescence peak of PbSe nanocrystals with the small particle size shifts to red and the photoluminescence peak of PbSe nanocrystals with the large particle size shifts to blue when the temperature increases. The photoluminescence intensity decreases and the full width at half-maximum gradually increase with the increase of temperature. This phenomenon is mainly attributed to the interaction between the exciton and phonon. According to the data fitting, optical phonon energy of 16.7meV in PbSe nanocrystals is almost independent on the particle size, approximately closed to the bulk material value.

Triplet Exciton Dissociation in Singlet Exciton Fission Photovoltaics

Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption.

Low-Temperature Approach to High-Yield and Reproducible Syntheses of High-Quality Small-Sized PbSe Colloidal Nanocrystals for Photovoltaic Applications

Small-sized PbSe nanocrystals (NCs) were synthesized at low temperature such as 50-80 °C with high reaction yield (up to 100%), high quality, and high synthetic reproducibility, via a noninjection-based one-pot approach. These small-sized PbSe NCs with their first excitonic absorption in wavelength shorter than 1200 nm (corresponding to size < ∼3.7 nm) were developed for photovoltaic applications requiring a large quantity of materials. These colloidal PbSe NCs, also called quantum dots, are high-quality, in terms of narrow size distribution with a typical standard deviation of ∼7-9%, excellent optical properties with high quantum yield of ∼50-90% and small full width at half-maximum of ∼130-150 nm of their band-gap photoemission peaks, and high storage stability. Our synthetic design aimed at promotion of the formation of PbSe monomers for fast and sizable nucleation with the presence of a large number of nuclei at low temperature. For formation of the PbSe monomer, our low-temperature approach suggests the existence of two pathways of Pb-Se (route a) and Pb-P (route b) complexes. Either pathway may dominate, depending on the method used and its experimental conditions. Experimentally, a reducing/nucleation agent, diphenylphosphine, was added to enhance route b. The present study addresses two challenging issues in the NC community, the monomer formation mechanism and the reproducible syntheses of small-sized NCs with high yield and high quality and large-scale capability, bringing insight to the fundamental understanding of optimization of the NC yield and quality via control of the precursor complex reactivity and thus nucleation/growth. Such advances in colloidal science should, in turn, promote the development of next-generation low-cost and high-efficiency solar cells. Schottky-type solar cells using our PbSe NCs as the active material have achieved the highest power conversion efficiency of 2.82%, in comparison with the same type of solar cells using other PbSe NCs, under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm(2).

Revealing the Exciton Fine Structure of PbSe Nanocrystal Quantum Dots Using Optical Spectroscopy in High Magnetic Fields

We measure the photoluminescence lifetime τ of excitons in colloidal PbSe nanocrystals (NCs) at low temperatures to 270 mK and in high magnetic fields to 15 T. For all NCs, τ increases sharply below 10 K but saturates by 500 mK. In contrast to the usual picture of well-separated "bright" and "dark" exciton states (found, e.g., in CdSe NCs), these dynamics fit remarkably well to a system having two exciton states with comparable--but small--oscillator strengths that are separated by only 300-900 μeV depending on NC size. Importantly, magnetic fields reduce τ below 10 K, consistent with field-induced mixing between the two states. Magnetic-circular dichroism studies reveal exciton g factors from 2-5, and magnetophotoluminescence shows >10% circularly polarized emission.

Effect of Air Exposure on Surface Properties, Electronic Structure, and Carrier Relaxation in PbSe Nanocrystals

Effects of air exposure on surface properties, electronic structure, and carrier relaxation dynamics in colloidal PbSe nanocrystals (NCs) were studied using X-ray photoelectron spectroscopy, transmission electron microscopy, and steady-state and time-resolved photoluminescence (PL) spectroscopies. We show that exposure of NC hexane solutions to air under ambient conditions leads to rapid oxidation of NCs such that up to 50% of their volume is transformed into PbO, SeO2, or PbSeO3 within 24 h. The oxidation is a thermally activated process, spontaneous at room temperature. The oxidation-induced reduction in the size of the PbSe "core" increases quantum confinement, causing shifts of the PL band and the absorption onset to higher energies. The exposure of NC solutions to air also causes rapid (within minutes) quenching of PL intensity followed by slow (within hours) recovery during which the PL quantum yield can reach values exceeding those observed prior to the air exposure. The short-term PL quenching is attributed to enhanced carrier trapping induced by adsorption of oxygen onto the NC surface, while the PL recovery at longer times is predominantly due to reduction in the efficiency of the "intrinsic" nonradiative interband recombination caused by the increase of the band gap in oxidized NCs. Although the analysis of subnanosecond relaxation dynamics in air-exposed NCs is complicated by a significant enhancement in fast carrier trapping, our picosecond PL measurements suggest that air exposure likely has only a weak effect on Auger recombination and also does not significantly affect the efficiency of carrier multiplication. We also show that the effects of air exposure are partially suppressed in PbSe/CdSe core/shell structures.

Solar Cells Based on Junctions between Colloidal PbSe Nanocrystals and Thin ZnO Films

We report a new type of excitonic solar cell based on planar heterojunctions between PbSe semiconductor nanocrystals and thin ZnO films. These solar cells generate large photocurrents and higher photovoltages compared to Schottky cells assembled with similar nanocrystal films. When illuminated with 100 mW/cm(2) simulated AM1.5 spectrum, these solar cells exhibit short-circuit currents between 12 and 15 mA/cm(2), open-circuit voltages up to 0.45 V, and a power conversion efficiency of 1.6%. The photovoltage depends on the size of the nanocrystals, increasing linearly with their effective band gap energy.

Site-Selective Optical Coupling of PbSe Nanocrystals to Si-Based Photonic Crystal Microcavities

A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed.

Surface Chemistry of Colloidal PbSe Nanocrystals

Solution nuclear magnetic resonance spectroscopy (NMR) is used to identify and quantify the organic capping of colloidal PbSe nanocrystals (Q-PbSe). We find that the capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the number of oleic acid ligands) is composed of tri- n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concentration. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and organic ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidation leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.

Size- and shape-controlled synthesis and properties of colloidal PbSe nanocrystals

In this work, it has been demonstrated that the co-ordinating tri- n -octylphosphine and oleic acid mixture plays an essential role in the control of the growth, size distribution, colloidal stability, crystallinity and shape of the resulting PbSe nanocrystals. A systematic combination of the reaction and growth conditions allows the possibility to produce high quality PbSe nanocrystals over a broad range of diameters with narrow size distributions without any size selective precipitation techniques being applied. The capping agent/ligands helps to control the grain growth of the nanocrystals during its formation and increases the dispersibility of the nanocrystals, which can easily be manipulated and used for various applications in optoelectronics and microscopy.

The Hidden Role of Acetate in the PbSe Nanocrystal Synthesis

Monodisperse spherical, star-shaped, and octahedral PbSe nanocrystals were synthesized via a hot injection method. We show that the shape and size of the colloidal PbSe nanocrystals are determined by the concentration of acetate and that only acetate-free reaction mixtures result in spherical nanocrystals. The presence of acetate leads to efficient oriented attachment of smaller PbSe nanoparticles along the 100 crystal axis. Comparing different synthesis procedures from the recent literature with our observations, we propose that the acetate, naturally present in insufficiently dried reaction mixtures, is responsible for many of the PbSe crystal shapes reported in the literature. In addition we show that it is possible to synthesize these star-shaped nanocrystals so monodisperse that they form ordered monolayers with crystal alignment.

Time-resolved intraband relaxation of strongly confined electrons and holes in colloidal PbSe nanocrystals

The relaxation of strongly-confined electrons and holes between 1P and 1S levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed.

Density of States Measured by Scanning-Tunneling Spectroscopy Sheds New Light on the Optical Transitions in PbSe Nanocrystals

The density-of-states function of individual colloidal PbSe nanocrystals varying in diameter between 3 and 7 nm is measured by resonant tunneling spectroscopy. It is in semiquantitative agreement with tight-binding calculations, but the energy separation between electron (hole) levels of S and P symmetry is systematically smaller than predicted by the theory. These results provide an explanation for the second and third excitonic optical transitions, which have been debated for a long time.

Optical Properties of Colloidal PbSe Nanocrystals

We present the structural and optical characterization of colloidal PbSe nanocrystals. The lowest-energy exciton transitions in these structures, with diameters between 3 and 8 nm, occur at wavelengths between 1.0 and 1.85 μm. Band-edge luminescence spectra with quantum yields as high as 80% are observed. Unexpectedly long (∼300 ns) luminescence lifetimes are observed. These properties are promising for applications in optoelectronics and microscopy.