# Raman Spectra of ZrS\({}_{2}\) and ZrSe\({}_{2}\) from Bulk to Atomically Thin Layers
## 1 Introduction  
Layered transition metal dichalcogenides (TMDCs) have recently attracted strong interest in the scientific community due to the new properties that may arise from the two-dimensional (2D) limit and their possible dependence on the number of layers [1]. Layered TMDCs are formed by the weak stacking of X-M-X sandwiches (where M is a transition metal and X is a chalcogen atom; see Figure 1). In this structure, the out-of-plane bonding (van der Waals forces) is much weaker than the in-plane one (covalent bonding), making these materials ideal for the isolation of thin layers and the study of dimensionality effects [2]. The archetypal example of properties dependence on dimensionality is graphene. In bulk (i.e., graphite), the motion of the charge carriers is described by the Schrodinger equation, whereas in the monolayer limit it is given by the Dirac equation [3]. However, the absence of a band gap in graphene makes it unsuitable for the realization of devices such as field effect transistors (FETs). In order to overcome this problem and complement graphene, semiconducting layered TMDCs of the group VIB (Mo, W) have acquired a relevant role [4]. Indeed, these materials present remarkable layer-dependent properties in the transition from an indirect band-gap (bulk) to a direct one (monolayer). Thus, with these 2D materials and their combination in heterostructures, it is possible to fabricate new electronic and optoelectronic devices such as flexible FETs, high-electron-mobility transistors (HEMTs), p-n junctions, alternative thin-film solar cells, or photodetectors, among others [5]. Nevertheless, despite their strong potential, to date most of the layered TMDCs remain almost or completely unexplored. For instance, recent experiments on TMDCs from group VB (Nb, Ta), which show metallic/superconductor behavior, have revealed opposite tendencies when approaching the 2D limit: a superconducting critical temperature (\(T_{\rm c}\)) suppression in 2H-NbSe\({}_{2}\)[6; 7; 8] and a \(T_{\rm c}\) enhancement in 2H-TaS\({}_{2}\)[9]. So far, the semiconducting TMDCs from group IVB (Ti, Zr, Hf) have not been investigated as much as their counterparts from group VIB. Recently, many-body states have been tuned in a monolayer of TiSe\({}_{2}\)[10] and, regarding HfX\({}_{2}\) (X = S, Se), thin films of HfS\({}_{2}\)[11; 12] and HfSe\({}_{2}\)[13] have been studied for their potential applications in FETs and phototransistors since it is theoretically predicted that monolayers of Zr and Hf TMDCs may exhibit higher mobility than group VIB ones [14]. In addition to the predicted higher mobility, it has been theoretically foretold that ZrX\({}_{2}\) monolayer may also exhibit an indirect-to-direct band gap transition tuned by strain [15; 16] and even a semiconductor-to-metal phase transition [17]. However, to the best of our knowledge, mechanically exfoliated thin layers of ZrX\({}_{2}\) have not been reported yet. There are few studies on thin layers of ZrX\({}_{2}\) and they are obtained by chemical exfoliation (nanosheets and nanodiscs) [18; 19], or grown by chemical vapor deposition (nanobelts and flakes) [20; 21; 22] or epitaxy [23]. Nowadays, Raman spectroscopy is a common and useful technique in the characterization of 2D materials [24] since the composition [25], polytypism [26], number of layers [27], strain [28], and the existence of defects [29] can be detected. ZrX\({}_{2}\) TMDCs present a 1T structure (i.e., one atom of zirconium is octahedrally coordinated by six atoms of chalcogen), as shown in Figure 1a,b, where a lateral view of a unit cell and the top view of four unit cells are represented, respectively. The structure is a trigonal crystal system with a P-3m1 space group and the unit cell is determined by \(\alpha=\beta=90^{\circ}\), \(\gamma=120^{\circ}\), and a = b = 3.663(2) A, c = 5.827(6) A for ZrS\({}_{2}\) and a = b = 3.772(2) A, c = 6.125(5) A for ZrSe\({}_{2}\)[30].