  The metal diborides are a class of ceramic materials with crystal structures
consisting of hexagonal sheets of boron atoms alternating with planes of metal
atoms held together with mixed character ionic/covalent bonds. Many of the
metal diborides are ultrahigh temperature ceramics like HfB$_2$, TaB$_2$, and
ZrB$_2$, which have melting points above 3000$^\circ$C, high mechanical
hardness and strength at high temperatures, and high chemical resistance, while
MgB$_2$ is a superconductor with a transition temperature of 39 K. Here we
demonstrate that this diverse family of non-van der Waals materials can be
processed into stable dispersions of two-dimensional (2D) nanosheets using
ultrasonication-assisted exfoliation. We generate 2D nanosheets of the metal
diborides AlB$_2$, CrB$_2$, HfB$_2$, MgB$_2$, NbB$_2$, TaB$_2$, TiB$_2$, and
ZrB$_2$, and use electron and scanning probe microscopies to characterize their
structures, morphologies, and compositions. The exfoliated layers span up to
micrometers in lateral dimension and reach thicknesses down to 2-3 nm, while
retaining their hexagonal atomic structure and chemical composition. We exploit
the convenient solution-phase dispersions of exfoliated CrB$_2$ nanosheets to
incorporate them directly into polymer composites. In contrast to the hard and
brittle bulk CrB$_2$, we find that CrB$_2$ nanocomposites remain very flexible
and simultaneously provide increases in the elastic modulus and the ultimate
tensile strength of the polymer. The successful liquid-phase production of 2D
metal diborides enables their processing using scalable low-temperature
solution-phase methods, extending their use to previously unexplored
applications, and reveals a new family of non-van der Waals materials that can
be efficiently exfoliated into 2D forms.
