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Posted: Dec 05, 2016

Large-yield synthesis of 2D antimonene nanocrystals

(Nanowerk Spotlight) Density functional theory computations have shown that monolayered arsenene and antimonene – which are indirect wide-band-gap semiconductors – under strain become direct band-gap semiconductors. Such dramatic transitions of electronic properties could open a new door for nanoscale transistors with high on/off ratio, blue/UV optoelectronic devices, and nanomechanical sensors based on new ultrathin semiconductors (Read more in our previous Nanowerk Spotlight: "Novel mono-elemental semiconductors: arsenene and antimonene join 2D family").
Following up on these theoretical predictions, researchers now have demonstrated two high-yield methods for fabricating antimonenes: A nanosheet solution by liquid exfoliation (Journal of the American Chemical Society, "Few-layer Antimonene: Large Yield Synthesis, Exact Atomical Structure and Outstanding Optical Limiting") and high-quality single crystals of few layer antimonene by CVD growth (Nature Communications, "Two-dimensional antimonene single crystals grown by van der Waals epitaxy").
"Our findings advance the field of nanocrystal inks in two aspects," Professor Haibo Zeng, Director of the Institute of Optoelectronics & Nanomaterials at Nanjing University of Science and Technology, tells Nanowerk. "Firstly, through simply heating a mixture of metal–organic precursors we were able to synthesize a wide range of transparent conducting oxide (TCO) nanocrystal inks, which can be assembled into high-performance electrodes."

High-yield one-pot process for oxide nanocrystal inks

This one-pot process can be used for various oxide nanocrystal inks with yields as high as 10 grams. The formed nanocrystals are of high crystallinity, uniform morphology, monodispersity, and high ink stability and feature effective doping.
"Secondly" adds Zeng, "the inks can be readily assembled into films with a surface roughness of 1.6 nm. Typically, a sheet resistance of 110 Ω/sq can be achieved with a transmittance of 88%, which is the best performance for TCO nanocrystal ink-based electrodes described to date."
These electrodes can thus drive a polymer light-emitting diode (PLED) with a luminance of 2200 cd m-2 at 100 mA cm-2. They could find applications in various solution-based, even flexible optoelectronics.
Antimonene polygons synthesized on mica substrates via van der Waals epitaxy
Antimonene polygons synthesized on mica substrates via van der Waals epitaxy. (a) Schematic illustration of the sample synthesis configurations. (b) Schematic diagram of van der Waals epitaxy. (c–f) Optical images of typical antimonene polygons with triangular, hexagonal, rhombic and trapezoidal shapes, respectively. The scale bar is 5 µm. (g) AFM image of a typical triangular antimonene sheet. The thicknesses are 4 nm. The scale bar is 1 mm. (h) AFM image of a tiny antimonene sheet. The thickness is ca. 1 nm. The scale bar is 50 nm. (© Nature Publishing Group) (click on image to enlarge)
In contrast to the previously TCO nanocrystal synthesis method, Zeng's team reports a facile and universal one-pot method for the synthesis of a wide range of TCO nanocrystal inks and the corresponding electrodes for all-solution-processed (ASP) devices with good performances.
The proposed approach is generic for various TCOs as well as other oxide (e.g., CoO, MnO, Fe3O4, CdO) nanocrystal inks.
"Our TCO nanocrystals are highly crystalline, with a uniform morphology and a narrow size distribution, as well as an effective doping control and a high colloidal stability over one year, making them suitable to be used as inks to print smooth, crack-free, highly transparent, and conductive films," notes Jianping Ji, a postgraduate researcher in Zeng's lab and first author of the paper in Nature Communications.

Transparent conducting oxide nanocrystal inks for transparent and flexible electrodes

TCO nanocrystal inks are compatible with flexible and stretchable substrates, which can be simply prepared by solution-based techniques, such as spin coating, ink-jet printing, spraying, and roll-to-roll production. The resulting high-performance transparent electrodes have a significant potential in low-cost and solution-based fields, such as organic light-emitting diodes (OLEDs), solar cells, photodetectors, wearable electronics, and smart electrochromic windows.
In recent years, the hot-injection method has become very popular for the synthesis of monodisperse colloidal nanocrystals, which have been used to prepare ITO and GZO nanocrystal inks.
"Nevertheless, such a process involves a chemical reaction between the injected sources and the mother solution and suffers from two disadvantages," explains Zeng. "First, a small dose of injection is usually chosen to achieve high-quality doping, but this injection mode leads to a low throughput and is difficult to scale-up. Second, the doping quality is very sensitive to the dopant element and the injection parameters. All of the parameters have to be changed to prepare oxide nanocrystal inks with different dopants. Thus, our motivation has been to develop a a one-pot and universal strategy for the fabrication of a wide range of TCO nanocrystal inks."
In order to be used as transparent conductive electrodes for flexible and stretchable optoelectronics in commercial applications, the main hurdle for TCO nanocrystal inks is lack of compatibility with current industrial solution-based process, such as roll-to-roll or blade coating production.
The researchers say that this could be solved by optimizing the viscosity and surface tension of the inks.
Aside from the problem of assembling films, the electrical properties of TCO nanocrystal electrodes should be further improved by short ligand exchanges or optimal post-treatments, which can meet the requirements for practical application.
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