Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media

Atomic layer deposition (ALD) is widely used for gas-phase deposition of high-quality dielectric, semiconducting, or metallic films on various substrates. In this contribution we propose the concept of colloidal ALD (c-ALD) for synthesis of colloidal nanostructures. During the c-ALD process, either nanoparticles or molecular precursors are sequentially transferred between polar and nonpolar phases to prevent accumulation of unreacted precursors and byproducts in the reaction mixture. We show that binding of inorganic ligands (e.g., S^2–) to the nanocrystal surface can be used as a half-reaction in c-ALD process. The utility of this approach has been demonstrated by growing CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of colloidal nanoheterostructures with mixed confinement regimes for electrons and holes. In these materials holes are confined to a thin (∼1.8 nm) two-dimensional CdSe quantum well, while the electron confinement can be gradually relaxed in all three dimensions by growing epitaxial CdS layers on both sides of the quantum well. The relaxation of the electron confinement energy caused a shift of the emission band from 510 to 665 nm with unusually small inhomogeneous broadening of the emission spectra

Selective Electrophoretic Deposition of CdSe Nanoplatelets

In the fields of nanoparticle synthesis and application, the control of the particle size, shape and composition is crucial. The tuning of these different parameters can be performed during the synthesis, but often, additional selection steps to improve the purity of a given nanoparticles’ population are necessary. These additional postsynthesis selection steps, that can include size selective precipitation, ultracentrifugation or liquid chromatography, are usually long and not well suited for a large quantity of materials. Here, we demonstrate that electrophoresis performed directly in organic solvent can be used to select and/or separate semiconductor nanoparticles according to their size and shape. In particular, we show that 2D nanoplatelets (NPL) can be very efficiently separated from spherical nanoparticles as the side product obtained during the NPL synthesis. The selectivity of the electrophoretic deposition we observe is mostly related to the nanoparticle surface charge. We show that centimeter scale, uniform film of nanoplatelets can be obtained even on nonconducting substrates. Compared to other methods this technique is fast, easy to implement and scalable, and should find various uses both in the fields of the nanoparticle synthesis and their applications

Electrolyte-Gated Colloidal Nanoplatelets-Based Phototransistor and Its Use for Bicolor Detection

Colloidal nanocrystals are appealing candidates for low cost optoelectronic applications because they can combine the advantages of both organic materials, such as their easy processing, and the excellent performance of inorganic systems. Here, we report the use of two-dimensional colloidal nanoplatelets for photodetection. We show that the nanoplatelets photoresponse can be enhanced by two to three orders of magnitude when they are incorporated in an all solid electrolyte-gated phototransistor. We extend this technique to build the first colloidal quantum dot-based bicolor detector with a response switchable between the visible and near-IR

Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets

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

Phonon Line Emission Revealed by Self-Assembly of Colloidal Nanoplatelets

We show that colloidal nanoplatelets can self-assemble to form a 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, greatly enhanced when the NPLs are self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton, and its presence in self-assembled nanoplatelets is explained using a model based on an efficient photons reabsorption between neighboring nanoplatelets. The presence of phonon replica at low temperatures in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets

Carrier Cooling in Colloidal Quantum Wells

It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators

Low Voltage, Hysteresis Free, and High Mobility Transistors from All-Inorganic Colloidal Nanocrystals

High-mobility solution-processed all-inorganic solid state nanocrystal (NC) transistors with low operation voltage and near-zero hysteresis are demonstrated using high-capacitance ZrO_<i>x</i> and hydroxyl-free Cytop gate dielectric materials. The use of inorganic capping ligands (In₂Se₄^2‑ and S^2‑) allowed us to achieve high electron mobility in the arrays of solution-processed CdSe nanocrystals. We also studied the hysteresis behavior and switching speed of NC-based field effect devices. Collectively, these analyses helped to understand the charge transport and trapping mechanisms in all-inorganic NCs arrays. Finally, we have examined the rapid thermal annealing as an approach toward high-performance solution-processed NCs-based devices and demonstrated transistor operation with mobility above 30 cm²/(V s) without compromising low operation voltage and hysteresis

Particle-Level Engineering of Thermal Conductivity in Matrix-Embedded Semiconductor Nanocrystals

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a “bottom-up” means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap

Two-Dimensional Growth of CdSe Nanocrystals, from Nanoplatelets to Nanosheets

We report the continuous lateral extension of cadmium selenide nanoplatelets into nanosheets using continuous injection of precursors at high temperature. We show that we can obtain CdSe nanosheets with lateral dimensions up to 700 nm and a well-defined thickness that can be tuned with atomic precision. When the nanosheets’ lateral size increases, they roll on themselves to form nanoscrolls that can unroll upon surface modification. The final geometry of the nanosheets can be tuned to different morphologies using precursors that favor the growth of specific crystal facets. We provide a detailed study of the CdSe nanosheets growth and its optimization for three different thicknesses

Real-Time in Situ Probing of High-Temperature Quantum Dots Solution Synthesis

Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to design new nanostructures and synthesize them in a predictive fashion. However, reliable data on the pathways leading from molecular precursors to nanocrystals are not available yet. We used synchrotron-based time-resolved <i>in situ</i> small and wide-angle X-ray scattering to experimentally monitor the formation of CdSe quantum dots synthesized in solution through the heating up of precursors in octadecene at 240 °C. Our experiment yields a complete movie of the structure of the solution from the self-assembly of the precursors to the formation of the quantum dots. We show that the initial cadmium precursor lamellar structure melts into small micelles at 100 °C and that the first CdSe nuclei appear at 218.7 °C. The size distributions and concentration in nanocrystals are measured in a quantitative fashion as a function of time. We show that a short nucleation burst lasting 30 s is followed by a slow decrease of nanoparticle concentration. The rate-limiting process of the quantum dot formation is found to be the thermal activation of selenium