Epitaxial Ni x Pd 1-x (111) Alloy Substrates with Continuously Tunable Lattice Constants for 2D Materials Growth.

Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow Nix Pd1-x (111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2 O3 and Cr2 O3 (grown as epitaxial films on α-Al2 O3 (0001)). The Cr2 O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with Nix Pd1-x [112̅] parallel to Cr2 O3 [112̅0], in contrast to growth on α-Al2 O3 that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated that the films are suitable as inexpensive and stable substrates for thin-film growth and for surface science studies. To demonstrate this suitability, bilayer SiO2 , a two-dimensional van der Waals material, was grown on a Nix Pd1-x (111) film tuned to closely match the film's lattice constant, with STM and electron diffraction results revealing a highly ordered, single-phase crystalline state.