NaSn₂As₂: An Exfoliatable Layered van der Waals Zintl Phase

Abstract

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties, enables the realization of unique physical phenomena in these few-atom thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = Group 1 or 2 element, Tt = Group 14 tetrel element and Pn = Group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant towards oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 {\Omega} m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics

Na1−xSn2P2 as a new member of van der Waals-type layered tin pnictide superconductors

“…NaSn 2 As 2 crystallizes in a trigonal R m unit cell, consisting of two layers of a buckled honeycomb network of SnAs, bound by the vdW forces and separated by Na ions 9 , as schematically shown in Fig. 1a,c.…”

“…1a,c. Because of the vdW gap between the SnAs layers, it can be readily exfoliated through both mechanical and liquid-phase methods 9,10 . Besides, the sister compound SrSn 2 As 2 , having a crystal structure analogous to NaSn 2 As 2 , has been theoretically suggested to be very close to the topological critical point, hosting three-dimensional Dirac state at the Fermi level 11 , which was experimentally investigated by angle-resolved photoemission spectroscopy 12 .…”

“…Besides, the sister compound SrSn 2 As 2 , having a crystal structure analogous to NaSn 2 As 2 , has been theoretically suggested to be very close to the topological critical point, hosting three-dimensional Dirac state at the Fermi level 11 , which was experimentally investigated by angle-resolved photoemission spectroscopy 12 . There are various structural analogues with conducting tin-pnictide (SnPn) layers, including Sn 4 Pn 3 13,14 and ASnPn 15–19 , as well as ASn 2 Pn 2 9,10,12,20–22 , where A denotes alkali or alkaline earth metal (see Fig. 1).…”

Evidence for s -wave pairing with atomic scale disorder in the van der Waals superconductor NaSn2As2

“…Previous angle-resolved photoemission spectroscopy (ARPES) experiments of NaSn 2 As 2 [9] have suggested a possible multiband electronic structure, which cannot be reproduced by simple first-principles band-structure calculations. The specific heat measurements reveal the bulk nature of superconductivity, but information on the superconducting gap structure, which is the key to the pairing mechanism, has not been obtained because of the lack of low-temperature measurements [1].…”

Exploring the versatility of liquid phase exfoliation: producing 2D nanosheets from talcum powder, cat litter and beach sand

“…26 These structures have been used in a wide range of applications from barrier composites 27,28 to battery electrodes 29,30 to photodetectors. 31,32 Importantly, LPE has been applied to a wide range of 2D materials including graphene, [33][34][35] 49 The large number of layered materials which have been exfoliated to nanosheets using LPE suggests an impressive degree of versatility. This is important as the greater the versatility of an exfoliation method, the more likely it is to be useful in practical terms.…”