Synthesis of Heterostructures of in4Se3/in Nanowires
Authors: Hsu An-Hsuan , Wang Chiu-yen 
Published: February 2020

Since the discovery of carbon nanotubes, many scientists have developed a numbers of nanomaterial (nanotubes, nanorods, nanowires) in order to minimizing the size of the devices. A large series of devices have been created on the basis of nanomaterials[1]. In addition, many properties of nanomaterials will be dramatically changed due to extremely small size or unique shape. For instance, electrical and optical properties, etc. That's why one-dimensional (1D) materials are important to nano-level photoelectric and electronic component. In the past decades, the In-Se system[2] has attracted to a great deal of attention in high-efficiency thermoelectric conversion due to its extraordinary adjustable electrical and thermal properties. The physical properties of indium selenide mainly depend on its intrinsic structural characteristics, including composition, phase, crystal structure. Structurally, indium selenide belongs to a composite system of different stoichiometry, including In4Se3
[3], In2Se3
[4], InSe[5], which is also used in electrochemical devices due to its low-dimensional structure. In general, nanowires are made by chemical vapor deposition (CVD) process. However, using CVD to obtain heterostructure is relatively complicated. Thus, injection molding process is a simple way to let nanowire's phase separated.

The topic of this research was divided into two main processes, the anodic treatment of aluminum sheet and In-Se system materials synthesized by vacuum melting, annealing and torch sintering, sequentially. The effects of melting and annealing process on the phase, composition, microstructure, of polycrystalline In4Se3/In were investigated. First, the high purity aluminum (99.999%) was carried out electrolytic- polishing, and then form nano-porous array of anodic aluminum oxide (AAO) by anodic oxidation process. After that, AAO was soaked in acid to get alumina template having aperture of 70~100 nm and thickness of 30 μm. It would be used for filling in functional In-Se one-dimensional (1D) materials. In the second part, the powder with 70 at% indium and 30 at% selenium were uniformly mixed in the quartz tube, and then melted into In0.7Se0.3 bulk in vacuum at high temperature. Then, the melting In0.7Se0.3 bulk was cast into AAO auxiliary template, a special high-temperature oven including In0.7Se0.3 bulk and AAO was heated up to melting point. Afterwards, press into the AAO template to form In + In4Se3/AAO composite by vacuum injection molding process. As-obtained samples were immersed in chromic acid to get heterostructure nanowires. Afterward these nanowires can be transformed into segmental structure of In4Se3/In by annealing. Moreover, the length ratio of In4Se3/In segmentation would be 4:1 via theoretical calculating. The morphology of the nanowires was observed by optical microscopy (OM). After annealing, the segmentations of In/In4Se3 nanowires were analyzed by field-emission scanning electron microscope (FE-SEM). It can be observed that the nanowires were obviously separated into In and In4Se3 parts. Using energy dispersive spectrum (EDS) to confirm the composition of the segment structure, the dark area of the nanowires was In4Se3 and the bright one was In. That is corresponding to the bulk composition of In4Se3 identified by Raman analysis. It has great potential that nanowire product will be applied in photovoltaic semiconductors.

References:

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[2]Jian-Jun Wang, Fei-Fei Cao, Lang Jiang, Yu-Guo Guo, Wen-Ping Hu and Li-Jun WanHigh. Performance Photodetectors of Individual InSe Single Crystalline Nanowire. Chem. Soc.131, 15602–15603, 2009.

[3]Han, Z.G. Chen, J. Drennan and J. Zou. Indium Selenides: Structural Characteristics, Synthesis and Their Thermoelectric Performances. Small, 10 2747-2767, 2014

[4]H. Hogg, H.H. Sutherland and D.J. Williams. The Crystal Structure of Tetraindium Triselenide. Acta Crystallographica Section B. 29, 1590, 1590-1593. 1973.

[5]Popovic, A. Tonejc, B. Grzeta-Plenkovic and R. Trojko. Revised and New Crystal Data for Indium Selenides. Journal of Applied Crystallogrphy, 12, 416-420. 1979.