A new precursor library yields high-quality quantum dots for device applications
Also see Report by
Hendricks
et al.
Colloidal semiconductor nanocrystals (NCs) or “quantum dots” offer exquisite ...control of absorption and emission colors for device applications by tuning their size and shape (
1
). The proliferation of NC applications has been enabled by the “hot-injection” synthesis, which can tightly control the particle size to achieve pure colors (
2
). However, current hot-injection synthesis recipes are largely incompatible with low-cost, large-area remote phosphor applications of NCs. Reagents are too expensive, reaction mixtures too dilute, and conversion yields too low, especially if size tuning is accomplished by adjusting the reaction time. On page 1226 of this issue, Hendricks
et al.
(
3
) report on a radical improvement of the hot-injection synthesis economics of metal sulfide NCs by introducing a library of inexpensive thioureas as reagents.
The possibilities offered by 1H solution NMR for the study of colloidal nanocrystal ligands are reviewed. Using CdSe and PbSe nanocrystals with tightly bound oleate ligands as examples, the solution ...NMR toolbox for ligand analysis is introduced, highlighting 1D 1H, diffusion ordered (DOSY) and nuclear Overhauser effect (NOESY) spectroscopy as NMR techniques that enable bound ligands to be distinguished from free ligands. For each of the toolbox techniques, it is outlined how dynamic stabilization in the fast exchange regime affects the spectra obtained. Next, it is shown how the perturbation of a purified dispersion by dilution or the addition of excess ligands can be used to analyze the binding of ligands to a nanocrystal. Finally, saturation transfer difference (STD) spectroscopy is presented as an NMR technique that may complement the established toolbox.
One of the greatest challenges in the field of semiconductor nanomaterials is to make trap-free nanocrystalline structures to attain a remarkable improvement of their optoelectronic performances. In ...semiconductor nanomaterials, a very high number of atoms is located on the surface and these atoms form the main source of electronic traps. The relation between surface atom coordination and electronic structure, however, remains largely unknown. Here, we use density functional theory to unveil the surface structure/electronic property relations of zincblende II–VI CdSe model nanocrystals, whose stoichiometry and surface termination agree with recent experimental findings. On the basis of the analysis of the surface geometry and the recent classification of the ligand surface coordination in terms of L-, X-, and Z-type ligands, we show that, contrary to expectations, most under-coordinated “dangling” atoms do not form traps and that L- and X-type ligands are benign to the nanocrystal electronic structure. On the other hand, we find clear evidence that Z-type displacement induces midgap states, localized on the 4p lone pair of 2-coordinated selenium surface atoms. We generalize our findings to the whole family of II–VI metal chalcogenide nanocrystals of any size and shape and propose a new schematic representation of the chemical bond in metal chalcogenide nanocrystals that includes explicitly the coordination number of surface atoms. This work results in a detailed understanding of the formation of surface traps and provides a clear handle for further optimization of colloidal nanocrystals for optoelectronics applications.
Given the current needs for lasers on flexible substrates or as disposable, low-cost appliances, solution-processable lasers based on tunable colloidal quantum dots (QD) could revolutionize the field ...of laser-based opto-electronics, much as these QD materials currently do for the growing markets of displays and lighting. In this perspective, we present the status of this rapidly advancing field, followed by a discussion of the remaining challenges and possible avenues for future research and valorization.
Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to ...nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.
We present synthesis protocols, based on indium halide and aminophosphine precusors, that allow for the economic, up-scaled production of InP quantum dots (QDs). The reactions attain a close to full ...yield conversion – with respect to the indium precursor – and we demonstrate that size tuning at full chemical yield is possible by changing the nature of the indium halide salt. In addition, we present ZnS and ZnSe shell growth procedures that lead to InP/ZnS and InP/ZnSe core/shell QDs that emit from 510 to 630 nm with an emission line width between 46 and 63 nm. This synthetic method is an important step toward performing Cd-free QDs, and it could help the transfer of colloidal QDs from the academic field to product applications.
The accessible emission spectral range of lead halide perovskite (LHP) CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) has remained so far limited to wavelengths below 1 μm, corresponding to the emission ...line of Yb3+, whereas the direct sensitization of other near-infrared (NIR) emitting lanthanide ions is unviable. Herein, we present a general strategy to enable intense NIR emission from Er3+ at ∼1.5 μm, Ho3+ at ∼1.0 μm and Nd3+ at ∼1.06 μm through a Mn2+-mediated energy-transfer pathway. Steady-state and time-resolved photoluminescence studies show that energy-transfer efficiencies of about 39%, 35% and 70% from Mn2+ to Er3+, Ho3+ and Nd3+ are obtained, leading to photoluminescence quantum yields of ∼0.8%, ∼0.7% and ∼3%, respectively. This work provides guidance on constructing energy-transfer pathways in semiconductors and opens new perspectives for the development of lanthanide-functionalized LHPs as promising materials for optoelectronic devices operating in the NIR region.
Colloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this ...work, short‐wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time‐resolved photoluminescence, carrier transport, and capacitance–voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD‐based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 µm, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 1012 Jones.
An optical model for lead sulfide quantum dot thin films is developed and applied on short‐wave infrared sensitive photodetectors. Through optical engineering, the cavity is optimized at the wavelength of choice, leading to significant boost in the device external quantum efficiency, achieving values higher than 30% at wavelengths above 1.4 µm.
Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic ...absorption coefficient μi of 4 to 11 nm sized colloidal CsPbBr3 nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μi is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr3 nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr3 sizing curve.
Transition metal dichalcogenides (TMDs) are gaining increasing interest in the field of lithium ion batteries due to their unique structure. However, previous preparation methods have mainly focused ...on their growth from substrates or by exfoliation of the bulk materials. Considering colloidal synthesis has many advantages including precision control of morphology and crystal phases, there is significant scope for exploring this avenue for active material formation. Therefore, in this work, we explore the applicability of colloidal TMDs using WSe2 nanocrystals for Li ion battery anodes. By employing colloidal hot-injection protocol, we first synthesize 2D nanosheets in 2H and 1T′ crystal phases. After detailed structural and surface characterization, we investigate the performance of these nanosheets as anode materials. We found that 2H nanosheets outperformed 1T′ nanosheets exhibiting a higher specific capacity of 498 mA h g−1 with an overall capacity retention of 83.28%. Furthermore, to explore the role of morphology on battery performance, 3D interconnected nanoflowers in 2H crystal phase were also investigated as an anode material. It is worth noting that a specific capacity of 982 mA h g−1 was exhibited after 100 cycles by these nanoflowers. The anode materials were characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.
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