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Low-toxicity copper chalcogenide semiconductor nanocrystals

Final Report Summary - LOTOCON (Low-toxicity copper chalcogenide semiconductor nanocrystals)

Nowadays nanomaterials are cornerstones of modern science and technology. Their fabrication by means of wet colloidal synthesis is a cheap alternative to lithographic techniques used for a production of nanosized electronic circuits. Furthermore, colloidal nanomaterials can find commercial use as the building blocks for inexpensive manufacturing of low cost and large area electronic and optoelectronic devices through solution-based processes, such as spin coating, dip coating, or inkjet printing, as compared to much more sophisticated and high equipment costs physical (chemical) vapor, atomic layer deposition and other techniques. A large variety of colloidal nanomaterials including metal, semiconductor and magnetic compounds were developed during last decades. Among them, semiconductor compounds based on copper chalcogenides have attracted significant interest owing to their potential in energy related applications, such as photovoltaics, thermoelectrics, and batteries. These materials are commonly made up of elements such as copper, zinc, sulfur, selenium that are basically earth abundant and relatively low-toxic. Thus, the development of the synthesis of alloyed copper chalcogenide based nanoparticles was a main focus of this project.
Based on the materials selection, the following general strategy for their fabrication, which includes two main steps, had been chosen: 1) a synthesis of binary copper chalcogenides with 2) subsequent partial cation exchange (i.e. partial replacement of Cu ions by guest cations, like Zn, In and Sn) aiming at complex alloyed nanomaterials with widely tunable composition and morphology. Therefore, the project implementation started with the development of easy scalable synthesis of the simplest copper chalcogenides, i.e. Cu2S(Se,Te) nanoparticles. These materials have an advantage of an easy control over their size, morphology and the crystal structure, as opposed to multicomponent systems. As a result of this step, a novel facile and general synthetic approach consisting of heating up of precursors in organic media has been developed. Employing this method allows for a synthesis of Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y nanocrystals at moderate temperatures, avoiding hot-injection and use of harmful and expensive phosphines, which currently are widely applied for the synthesis of cadmium chalcogenides. Results of this work have been published in Chem. Mater. 2014, 26, 1442−1449. Furthermore, control of the reaction conditions (temperature regime, stabilizers and solvents employed, precursor ratios, etc.) results in an efficient shape control from small (4-10 nm) spherical particles to large and thin 2D nanosheets (lateral dimensions up to 10 μm, thickness up to 50 nm). The latter are the nanoscale building blocks that are possibly most suitable for electrically conducting thin films while retaining the opportunity of solution processing. Nanosheets have structure comparable to atomic layers made by other conventional deposition techniques such as molecular beam deposition. Moreover, within this work package copper sulfide nanostructured assemblies, prepared using a recently developed facile synthesis, have been utilized as cathodes for fabrication and testing of lithium-ion batteries. Results of this study will be published elsewhere. An important advantage of synthetic approaches developed is their easy scalability (gram-amounts are achieved thus far using a laboratory equipment) beneficial for a potential industrial application.
Having developed a scalable and controllable synthesis of copper chalcogenides, next step has been taken towards the development of an in-situ partial cation exchange aiming at ternary and quaternary copper chalcogenide-based compounds, such as ternary/quaternary copper-zinc(tin)-selenide(sulfide) nanomaterials with tunable composition and morphology. This work has resulted in a convenient approach for the preparation of alloyed nanoplates with tunable band gap. In this method the initial feed ratio of cation precursors (i.e. Zn and/or Sn) to copper ions appears to mediate the final composition of the alloyed compounds without altering the morphology and crystal structure as that of the starting copper selenide-sulfide plates. Details of the work are published in ACS Nano 2014, 8, 8407-8418 (open access paper). Importantly, the strategy proposed can be extended to other cations, as well as other copper chalcogenide nanoparticles, i.e. copper sulfide and telluride, with various shapes and crystal structures, or to design new chemical compounds with metastable crystal phases yielding more complex tailor-made nanomaterials with tunable properties, which otherwise would be more difficult to access from the direct synthesis. We demonstrated that by the incorporation of guest zinc and tin cations into selenide/sulfide anion framework of Cu2-xSeyS1-y nanoplates, it is possible to engineer the band gap of the resulting alloyed materials, which opens new opportunities for their application as light absorbers to fabricate solar cells.
The partial cation exchange route developed had been successfully applied to much larger copper selenide nanosheets obtained in the first part of the project. These materials have been processed into thin films with subsequent implementation of solar cell devices made thereof. The paper summarizing results of this work is in preparation and will be published elsewhere. In addition, using partial cation exchange, a novel method for the synthesis of ternary and quaternary copper-indium(zinc)-sulfide nanoparticles has been proposed. Cu-In-Zn-S nanoparticles are very promising alternative to well established cadmium chalcogenide quantum dots aimed to use in various light emissive applications, such as light emitting diodes, lasers, displays, detectors, biomarkers, etc. In our method, obtained Cu2S nanocrystals underwent the partial cation exchange in which indium ions replaced copper with preservation of the shape and crystal structure of the initial material. Sequentially, zinc ions were introduced and formed a gradient alloy (CuInS2)ZnS nanocrystals. Furthermore, it was shown that by combining indium- and zinc-precursors in one pot, homogenously alloyed Cu-In-Zn-S particles with higher zinc content could be synthesized. The composition of the particles can be well controlled by varying the reaction time as well as the ratio between guest cations in solution and copper in nanocrystals. Moreover, it was demonstrated that the sequential exchange with zinc leads to a sufficient increase of the photoluminescence efficiency and that light emission could be tuned from 850 to 1030 nm by carefully controlling the Cu:In:Zn content in the NCs. Cytotoxicity tests confirmed biocompatibility of Cu-In-Zn-S particles synthesized, which opens up opportunities for their application as fluorescent markers in biolabeling. The paper discussing results of this investigation is currently in preparation and will be published elsewhere. Therefore, the partial cation exchange strategy developed in this project paves a way to practically unlimited number of alloyed/doped nanomaterials exhibiting a wide variety of physico-chemical properties.
The development of complex alloyed nanomaterials using partial cation exchange reactions inspired another work within this project, and namely the detailed investigation of the mechanism of cation exchange reactions. Although currently cation exchange in nanomaterials is being intensively studied and used, little is known about its mechanism. Therefore, interactions of host-guest cation couples, restructuring of crystal lattices of colloidal nanoparticles, the role of cation vacancies have been elucidated employing two types of copper selenide nanoparticles, i.e. stoichiometric (nonvacant) Cu2Se and vacant Cu2-xSe by exchange of copper to silver, zinc, cadmium, led and other cations. In this work we have demonstrated that vacancies play a role of guest-cation carriers significantly accelerating exchange reactions and pushing this transformation to a greater extent, as compared to the fully stoichiometric compound. Furthermore, depending on reaction conditions, different types of structures can be prepared, such as dimer-like and core/shells. A paper discussing results is in preparation. This work had branched into another interesting sub-project on in-situ optical investigation of electrically driven surface plasmon resonance of copper selenide nanocrystals by combining cyclic voltammetry and absorption optical spectroscopy for tuning and measuring the plasmon band of Cu2-xSe nanocrystals embedded into a conducting polymer matrix. These results will be published in an outcoming paper.
Overall, the results obtained in the project can be interesting to a broad researchers’ community, including chemists, material scientists, physicists, biologists. Moreover, owing to easy scalability of the synthetic approaches developed, this work might be of interest to industrial engineers, dealing with development of solar cells, thermoelectric materials, and lithium-ion batteries. In addition, within last few years alloyed copper chalcogenide nanomaterials have also been successfully employed in catalysis. All above mentioned research lines of the project yielded interesting results which are in preparation for publication. In addition to two already published papers, five research articles are currently in preparation and will be published within next few months. During the project implementation the research fellow supervised and tutored a PhD student (the work on Cu2−xS, Cu2−xSeyS1−y, Cu2−xTeyS1−y nanocrystals and CuSe nanosheets) and a visiting PhD student (work on assembled copper sulfide nanoparticles and their use in lithium-ion batteries). Also he directly supervised an Erasmus project (work on Cu-Zn-In-S nanocrystals). Moreover, his background in water based colloidal synthesis and ligand exchange techniques gained at the Dresden Technical University helped to fabricate self-assembled micro-lasers via the “coffee-ring effect” that display single-mode operation and a very low threshold (a paper accepted in Small).