Strategies for enhancing conductivity of colloidal nanocrystals and their photoelectronic applications

Semiconductor colloidal nanocrystals(NCs)have size-and shape-dependent optoelectronic properties due to the quantum confinement effect,and are considered to be promising optoelectronic materials.Among them,Ⅱ-Ⅵ(CdSe,CdS,CdTe,etc.)andⅣ-Ⅵ(PbSe,PbTe,PbS,etc.)have been widely studied as representative colloidal NCs.However,the surfactant used in its synthesis progress results in the NCs surface covered by an insulating shell,which greatly affects the exciton separation and carrier transport of colloidal NCs-based photovoltaic devices.Therefore,how to design high-efficiency optoelectronic devices by improving the transport performance of carriers has been a great challenge.The key issues in the research ofⅡ-Ⅵ(CdSe,CdS,CdTe,etc.)andⅣ-Ⅵ(PbSe,PbTe,PbS,etc.)colloidal NCs were summarized,including synthesis strategy,morphology/size adjustment,surface ligand design,improvement of conductivity and their optoelectronic properties.The influence of surface ligands on the stability and dispersion of NCs was firstly introduced,and then strategies of improving electrical conductivity of NCs were discussed,such as ligands exchange,doping,self-assembly and plasmons,which provided a good foundation for the subsequent preparation of optoelectronic devices.The future development direction of NCs optoelectronic devices is expounded from the aspects of materials composition,comprehensive preparation and flexible processing of colloidal NCs.

Efficient up-conversion photoluminescence in all-inorganic lead halide perovskite nanocrystals

Up-conversion photoluminescence(UCPL)refers to the elementary process where low-energy photons are converted into high-energy ones via consecutive interactions inside a medium.When additional energy is provided by intermnal thermal energy in the form of lttice vibrations(phonons),the process is called phonon-assisted UCPL.Here,we report the exceptionally large phonon-assisted energy gain of up to^8kgT(kg is Boltzmann constant,T is temperature)on all-inorganic lead halide perovskite semiconductor colloidal nanocrystals that goes beyond the maximum capabilty of only harvesting optical phonon modes.By systematic optical study in combination with a statistical probability model,we explained the nontrivial phonon-assisted UCPL process in perovskites nanocrystals,where in addition to the strong electron-phonon(light-matter)coupling,other nonlinear processes such as phonon-phonon(matter-matter)interaction also effectively boost the up-conversion efficiency.

Colloidal CsPbBr3 perovskite nanocrystal films as electrochemiluminescence emitters in aqueous solutions

Perovskite nanocrystals （NCs）, which have emerged as a new class of phosphors with superb luminescence properties and bandgaps that can be easily tuned using chemical methods, have generated tremendous interest for a wide variety of applications where colloidal quantum dots have been very successful as carrier sources. In this study, self-assembled films of CsPbBr3 NCs were produced via drop casting of colloidal NCs onto glassy carbon electrodes （GCEs） to form an NC film-modified electrode. The possible fabrication process of the CsPbBr3 NCs films was discussed. We further studied the anodic electrochemiluminescence （ECL） behavior of the perovskite CsPbBr3 NCs film using cyclic voltammetry with tripropylamine （TPA） as a coreactant, and a possible ECL mechanism was proposed. Briefly, TPA was oxidized to produce strongly reducing radical spedes, which can react with electrochemically oxidized CsPbBr3 NCs to generate excited CsPbBr3 NCs＊ capable of light emission. The relative stability of the ECL emission of the CsPbBr3 NC films under aqueous conditions was also investigated, and it was found that they showed operational stability over the first three hours, indicating suitable reliability for application as sensing materials. The results suggested that semiconducting perovskite NCs have great potential for application in the ECL field.

Shell-thickness dependent optical properties of CdSe/ CdS core/shell nanocrystals coated with thiol ligands

Using CdSe/CdS core/shell nanocrystals with 1-10 monolayers of CdS shell as the model system, we studied effects of thiol ligands on optical properties of the nanocrystals. The core/shell nanocrystals with original ligands possessed near unity photoluminescence （PL） quantum yield and single-exponential PL decay dynamics. The effects of thiol ligands on optical properties were found to depend on the shell thickness, environment （with/without oxygen）, and excitation power （single- or multi-exciton）. Systematic and quantitative results reported in this work should provide necessary information for fundamental understanding and technical applications of quantum dots （QDs） coated with thiol ligands.

Efficient quasi-stationary charge transfer from quantum dots to acceptors physically-adsorbed in the ligand monolayer

Alkanoate-coated CdSe/CdS core/shell quantum dots(QDs)with near-unity photoluminescence(PL)quantum yield and monoexponential PL decay dynamics are applied for studying quasi-stationary charge transfer from photo-excited QDs to quinone derivatives physically-adsorbed within the ligand monolayer of a QD.Though PL quenching efficiency due to electron transfer can be up to>80%,transient PL and transient absorption spectra reveal that the charge transfer rate ranges from single-digit nanoseconds to sub-nanoseconds,which is~3 orders of magnitude slower than that of static charge transfer and〜2 orders of magnitude faster than that of collisional charge transfer.The physically-adsorbed acceptors can slowly(500-1,000 min dependent on the size of the quinone derivatives)desorb from the ligand monolayer after removal of the free acceptors.Contrary to collisional charge transfer,the efficiency of quasi-stationary charge transfer increases as the ligand length increases by providing additional adsorption compartments in the elongated hydrocarbon chain region.Because ligand monolayer commonly exists for a typical colloidal nanocrystal,the quasi-stationary charge transfer uncovered here would likely play an important role when colloidal nanocrystals are involved in photocatalysis,photovoltaic devices,and other applications related to photo-excitation.

Inkjet printing of epitaxially connected nanocrystal superlattices

Access to a blossoming library of colloidal nanomaterials provides building blocks for complex assembled materials.The journey to bring these prospects to fruition stands to benefit from the application of advanced processing methods.Epitaxially connected nanocrystal(or quantum dot)superlattices present a captivating model system for mesocrystals with intriguing emergent properties.The conventional processing approach to creating these materials involves assembling and attaching the constituent nanocrystals at the interface between two immiscible fluids.Processing small liquid volumes of the colloidal nanocrystal solution involves several complexities arising from the concurrent spreading,evaporation,assembly,and attachment.The ability of inkjet printers to deliver small(typically picoliter)liquid volumes with precise positioning is attractive to advance fundamental insights into the processing science,and thereby potentially enable new routes to incorporate the epitaxially connected superlattices into technology platforms.In this study,we identified the processing window of opportunity,including nanocrystal ink formulation and printing approach to enable delivery of colloidal nanocrystals from an inkjet nozzle onto the surface of a sessile droplet of the immiscible subphase.We demonstrate how inkjet printing can be scaled-down to enable the fabrication of epitaxially connected superlattices on patterned sub-millimeter droplets.We anticipate that insights from this work will spur on future advances to enable more mechanistic insights into the assembly processes and new avenues to create high-fidelity superlattices.