# Synthesis and Characterization of Highly Crystalline Vertically Aligned WSe\({}_{2}\) Nanosheets
## 1 Introduction  
Tungsten diselenide (WSe\({}_{2}\)) belongs to the transition-metal dichalcogenides (TMDs) family. The chemical formula of TMDs is MX\({}_{2}\), where M is a transition metal (Mo, W, Nb, Ta, etc.) and X is a chalcogenide (S, Se, etc.). The MX\({}_{2}\) compounds form layers composed of three atomic planes, namely: one metal atom between two chalcogenide atoms, covalently bonded, and the layers are linked between them by van der Waals forces [1]. Here, this is the case of the material of interest, WSe\({}_{2}\)[2]. Bulk WSe\({}_{2}\) has attracted increasing attention because of its interesting properties, exemplified by its ultralow thermal conductivity at room temperature (0.05 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)) when considering disordered crystals [3]. This bulk material has also been reported to be efficient in different applications, such as catalysts for hydrogen evolution reactions [4; 5], or in photovoltaic devices [6]. However, the recent craze for this material comes from the possibility of obtaining TMDs as 2D materials, similar to graphene, but with complementary electronic properties; these are indeed semi-conductors [7]. Actually, it has been reported that the properties of TMDs, such as MoS\({}_{2}\), WS\({}_{2}\), and WSe\({}_{2}\), depend directly on the number of layers in the structure, especially in systems with few layers. [7]. Moreover, monolayer WSe\({}_{2}\) possesses a small band-gap (smaller than monolayer MoS\({}_{2}\)), and shows an ambipolar transport phenomenon [8]. Several potential optoelectronic applications have been described using monolayer WSe\({}_{2}\), such as photodetectors [9], light-emitting diodes [10], and solar-energy convertors [11].  
The attractive reported properties and potential applications of WSe\({}_{2}\) materials require a well-controlled synthesis, in terms of their structure, including the number of layers, crystallographicphase composition, or/and film morphology. For this purpose, different synthesis techniques, such as chemical-vapor transport using a sealed ampoule containing W and Se materials under a vacuum and heated at a high temperature [4; 12; 13; 14], chemical and mechanical exfoliation [15; 16], physical techniques (molecular beam epitaxial growth [17], pulsed laser deposition [18], and magnetron sputtering of W in an Se-rich atmosphere [6]), chemical approaches (colloidal method [19; 20] and electrodeposition [21]), and atmospheric pressure chemical vapor deposition (APCVD) [22; 23; 24; 25; 26], have been used to obtain WSe\({}_{2}\). The majority of recent studies are focused on the synthesis of 2D WSe\({}_{2}\), with domains of different shapes and sizes. Furthermore, only a few are dedicated to thin films' synthesis with a controlled morphology, orientation, and crystallography, although these characteristics depend on the production method and growth parameters [14; 23; 27]. We report here on a simple growth strategy using the APCVD technique to obtain vertically-aligned WSe\({}_{2}\) nanosheets by rapid selenization. The developed method is an atmospheric pressure system technique that is rapid, scalable, and cost-effective, and has been applied previously for aligned MoS\({}_{2}\) nanosheets [28]. The produced samples were characterized using usual techniques such as electron microscopies, photoluminescence (PL), UV-visible and Raman spectroscopies, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).