Semiconductors are at the basis of electronics. Up to now, most devices that contain semiconductors use materials obtained from a top down approach with semiconductors grown by molecular beam epitaxy ...or chemical vapor deposition. Colloidal semiconductor nanoparticles have been synthesized for more than 30 years now, and their synthesis is becoming mature enough that these nanoparticles have started to be incorporated into devices. An important development that recently took place in the field of colloidal quantum dots is the synthesis of two-dimensional (2D) semiconductor nanoplatelets that appear as free-standing nanosheets. These 2D colloidal systems are the newborn in the family of shaped-controlled nanoparticles that started with spheres, was extended with rods and wires, continued with tetrapods, and now ends with platelets. From a physical point of view, these objects bring 1D-confined particles into the colloidal family. It is a notable addition, since these platelets can have a thickness that is controlled with atomic precision, so that no inhomogeneous broadening is observed. Because they have two large free interfaces, mirror charges play an important role, and the binding energy of the exciton is extremely large. These two effects almost perfectly compensate each other, it results in particles with unique spectroscopic properties such as fast fluorescent lifetimes and extreme color purity (narrow full width at half-maximum of their emission spectra). These nanoplatelets with extremely large confinement but very simple and well-defined chemistry are model systems to check and further develop, notably with the incorporation in the models of the organic/inorganic interface, various theoretical approaches used for colloidal particles. From a chemical point of view, these colloidal particles are a model system to study the role of ligands since they have precisely defined facets. In addition, the synthesis of these highly anisotropic objects triggered new research to understand at a mechanistic level how this strong anisotropy could be generated. Luckily, some of the chemical know-how built with the spherical and rod-shaped particles is being transferred, with some adaptation, to 2D systems, so that 2D core/shell and core/crown heterostructures have recently been introduced. These objects are very interesting because they suggest that multiple quantum wells could be grown in solution. From the application point of view, 2D colloidal nanoplatelets offer interesting perspectives when color purity, charge conductivity, or field tunable absorption are required. In this Account, we review the chemical synthesis, the physical properties, and the applications of colloidal semiconductor nanoplatelets with an emphasis on the zinc-blende nanoplatelets that were developed more specifically in our group.
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We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements. Excitation ...densities as high as an average of 18 electron–hole pairs per nanoplatelet were reached. Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller. The biexciton Auger recombination rate in nanoplatelets is more than 1 order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes, and photovoltaics.
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The optical and optoelectronic properties of colloidal quantum dots strongly depend on the passivation of their surface. Surface states are however difficult to quantify using optical spectroscopy ...and techniques based on back gated field effect transistors are limited in the range of carrier density that can be probed, usually significantly below one charge carrier per particle. Here we show that electrolyte gating can be used to quantitatively analyze the increase of defects in a population of nanoparticles with increasing surface irregularities. We illustrate this method using CdSe nanoplatelets that are grown in their thickness using low temperature layer-by-layer method. Spectroscopic analysis of the samples confirm that the nanoplatelet thickness is controlled, on average, with atomic precision, but structural analysis with transmission electron microscopy shows that the number of surface defects increases with the nanoplatelet thickness. The amount of charge defects is probed quantitatively using electrolyte-gated field effect transistor (EFET). We observe that the threshold voltage of the EFET increases with the NPL thickness, in agreement with the structural analysis. All samples displayed n-type conduction with strong current modulation (subthreshold swing slope of 100 mV/decade and on/off ratio close to 107). We also point out that an efficient electrolyte gating of the film requires a fine control of the nanoparticle film morphology.
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A new unsymmetrical geminal dianion that contained both a phosphine oxide moiety and a phosphine sulfide moiety has been synthesized. Its reactivity towards RuII was explored, which led to the ...formation of a highly reactive carbene complex that evolved at room temperature to yield a kinetic orthometalated RuII complex through CH activation of the phenyl group of the phosphine oxide moiety. This insertion was found to be thermally reversible and a second CH insertion occurred at a phenyl group of the phosphine sulfide moiety to form the thermodynamic orthometalated RuII complex in a diastereospecific manner. DFT calculations fully rationalized the experimental findings in terms of the relative energies of the kinetic and thermodynamic products and allowed the mechanism of this process to be fully understood.
PS, you're a gem: A new type of geminal dianion that combines PO and PS moieties allows the synthesis of a highly reactive Ru–carbene complex (see scheme). This complex exhibits a diastereospecific intramolecular CH activation of the ligand.
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A series of mixed tetradentate ligands associating two iminophosphorane moieties with two phosphino, thiophosphino, or phosphine oxide groups (labelled 2, 3, and 4 respectively) have been prepared ...from the corresponding aminophosphonium derivatives. Their coordination to the iron dichloride metal fragment was achieved using the FeCl(2)(THF)(1.5) precursor leading to (P(2)N(2))FeCl(2) (5), (S(2)N(2))FeCl(2) (6), and (O(2)N(2))FeCl(2) (7). These complexes were shown to be paramagnetic. Moreover, those ligands can act as bi, tri or tetradentate, as evidenced by X-ray structure analysis of the complexes, depending on the nature of the pendant PY coordinating ligand (Y = lone pair, S, O). Indeed, while only one phosphorus atom is coordinated to iron in 5 (PNN coordination), no thiophosphine moiety is connected to Fe in 6 (NN coordination), whereas both phosphine oxide arms are linked to the metal in 7 (ONNO) coordination. For ligand 2, coordination reactions were also performed with a non-chlorinated iron precursor (either Fe(CH(3)CN)(6)(BF(4))(2) or Fe(H(2)O)(6)(BF(4))(2)) leading to complexes 8 and 9 depending on the reaction conditions. The different iron(II) complexes 5-9 were tested in catalytic transfer hydrogenation of acetophenone, and were found to be efficient for reactions carried out at 82 degrees C.
We have synthesized atomically flat CdSe/CdTe core/crown nanoplatelets (NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control of the crown lateral dimensions. In these type-II NPLs, the ...charges separate spatially, and the electron wave function is localized in the CdSe core while the hole wave function is confined in the CdTe crown. The exciton’s recombination occurs across the heterointerface, and as a result of their spatially indirect band gap, an important emission red shift up to the near-infrared region (730 nm) is observed with long fluorescence lifetimes that range from 30 to 860 ns, depending on the type of interface between the core and the crown. These type-II NPLs have a high quantum yield of 50% that can be further improved to 70% with a gradient interface. We have characterized these novel CdSe/CdTe core/crown NPLs using UV–vis, emission, and excitation spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy.
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We study the synthesis of indium phosphide quantum dots using InCl3, tris(dimethylamino)phosphine (P(NMe2)3), and oleylamine. We optimized the reaction conditions to reach high chemical yield ...(∼70%) and size control of the quantum dots with absorption maxima over all the visible range. Kinetic studies of the formation of the quantum dots show that, under certain conditions, the growth of nanoparticles seems to approach a LaMer type growth. We have used 31P NMR, mass spectroscopy, and DFT calculations to decipher the reaction mechanisms of InP formation at the molecular level. The mechanistic investigation is in good agreement with the conclusions drawn from the optimization of the synthetic conditions.
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We present the synthesis of a novel type of chalcogenide nanoplatelets. Starting from CdS core or CdSe/CdS core/shell nanoplatelets, we use sequential cation exchange to copper and then to either ...zinc or lead to obtain ZnS and PbS core or ZnSe/ZnS and PbSe/PbS core/shell structures. The procedure preserves well the 2D geometry of the nanoplatelets, provided that they are more than 6 monolayers (∼1.8 nm) thick. The core/shell structure is also well conserved during the cation exchange as verified by TEM images. The nanoplatelets exchanged with Zn crystallize in a zinc blende structure, like the initial Cd-based material, whereas when Pb is used, the final nanoplatelets have a rock-salt crystal structure. We explored the copper cation exchange process using energy dispersive X-ray spectrometry with 1 nm resolution on nanoplatelets standing on their edges, and we show that copper ions diffuse uniformly from the outside of the nanoplatelet to the inside during the exchange.
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The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are ...studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron–hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a defect site. The electron that remains in the nanoplatelet moves in the Coulomb potential provided by the trapped hole.
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We report a simple, low temperature and scalable synthesis of wurtzite ZnS anisotropic nanocrystals performed under air. The reaction between a zinc salt (nitrate or chloride) with thioacetamide and ...amines at 60 °C yields ZnS nanoplatelets or nanowires absorbing in the ultra-violet region with a full width at half maximum between 11 nm and 15 nm.
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