  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.
