Surface Instability and Chemical Reactivity of ZrSiS and ZrSiSe Nodal-Line Semimetals

Among topological semimetals, in nodal-line semimetals (NLSM) conduction and valence bands cross each other. In particular, in NLSM, topological constraints protect band crossings and, moreover, band touching forms nodal lines or rings. Recently, topological nodal lines have been observed in bulk ZrSiX compounds (X = S, Se, Te). In ZrSiX, a tetragonal structure is formed by the stacking of X-ZrSi-Zr-X slabs covalently bonded between each other, whose strength decreases by replacing S with Se or Te ions. This class of materials exhibits large and non-saturating magnetoresistance and ultrahigh mobility of charge carriers. 
The control over surface phenomena, including oxidation, degradation, and surface reconstruction is a crucial step in order to evaluate the feasibility of the exploitation in technology of ZrSiX. 
By means of X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline, high-resolution electron energy loss (HREELS) and density functional theory, an international team of researchers from Italy, China, Russia, Taiwan, and USA (coordinated by University of L’Aquila) has studied the evolution of ZrSiS and ZrSiSe surfaces in oxygen and ambient atmosphere.
The chemical activity of ZrSiX compounds is mainly determined by the interactions of Si layer with ZrX sublayer. Any adsorption provides distortion of the Si layer (flat in bulk). In the case of ZrSiS, the ZrS sublayer is almost the same as in bulk and therefore adsorption is unfavorable because it provides distortions of Si layer. In the case of ZrSiSe, the ZrSe sublayer is already strongly distorted (structure different from bulk), and, therefore, further distortion of Si layer by adsorption is favorable (Figure 1). 
 

Figure 1.    Atomic structure of different steps of the process of the oxidation of ZrSiSe from (a-d) Zr-sites and (e-h) Si-sites. Red, light blue, black and yellow balls represent O, Zr, Se, and Si atoms, respectively. On panels (a) and (e) physical adsorption of single oxygen molecule is depicted. Panels (b) and (f) represent the situation of uniform coverage of the surfaces by molecular oxygen. In panels (c) and (g), decomposition of single oxygen molecule on the surfaces is represented. Panels (d) and (h) show total oxidation of the surfaces.
 

The pristine, undefected ZrSiS surface is quite inert toward oxygen and, correspondingly, the sticking coefficient of oxygen is estimated to be less than 10⁻³at room temperature. Surface oxidation is achieved only for ZrSiS with a considerable amount of defects introduced by ion sputtering. On the other hand, ZrSiSe has a much stronger reactivity toward oxygen. The ZrSiSe surface exposed to different doses of O₂or directly to air has similar vibrational spectra, core levels (Figure 2) and valence band, thus indicating that it is particularly prone to oxidation. As a matter of the fact, both experiments and theory demonstrate that, while the ZrSiS has a termination layer with both Zr and S atoms, surface reconstruction occurs in ZrSiSe with the appearance of Si surface atoms.

Figure 2.  Core-level spectra for ZrSiS (left) and ZrSiSe (right). Black, red, and blue curves correspond to as-cleaved, O2-dosed and air-exposed samples, respectively. The photon energy is 800 eV.

Two well-distinct scenarios concerning the reconstruction of Zr-terminated surface are feasible. The first scenario corresponds with the formation of dangling bonds between O and Zr atoms. In this case, Zr⁴⁺ions form two additional bonds with X atom from the same layer, with the subsequent breaking of Zr-X bonds with the sublayer and the resulting decoupling of ZrOX layer. Another scenario is related to the formation of O bridges between two Zr atoms on the surface. The predominance of one or another scenario of the reconstruction of Zr-terminated surface depends on the specific ZrSiX compound and it is directly associated to the different distortions and interlayer distances. The strong distortions of the surface layers and the possibility of the detachment of surface layers makes the use of ZrSiX as catalysts rather problematic, since poisoning of the catalyst seems unavoidable.
Both experimental and theoretical results suggest that it is crucial to protect the surfaces of ZrSiX materials. In contrast to the case of black phosphorus and other layered materials, an appropriate capping layer for ZrSiX should combine i) the prevention of the interaction of the surface with oxidative species and ii) the saturation of dangling bonds with minimal distortion of the surface.

This research was conducted by the following research team:

Danil W. Boukhvalov¹, Raju Edla², Piero Torelli², Raman Sankar³, Chia-Nung Kuo⁴, Chin Shan Lue⁴, Yanglin Zhu⁵, Zhiqiang Mao⁵, Jin Hu⁶, Luca Ottaviano⁷,Antonio Politano⁷

¹ Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, P. R. China
² CNR-IOM, Trieste, Italy
³ Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan
⁴ Department of Physics, National Cheng Kung University, Tainan, Taiwan
⁵ Department of Physics, Pennsylvania State University, USA
⁶ Department of Physics, Institute for Nanoscience and Engineering, University of Arkansas, USA
⁷ Università degli Studi dell’Aquila, Italy
 

Contact persons:

Antonio Politano, e-mail:
Piero Torelli, e-mail: