2D Amorphous Materials and their Properties:
From Ultra-Low k Dielectrics to Ferromagnetism in Co doped 2D NbSe2

Disordered systems in the atomic limit offer several existing possibilities in both basic science and applications
which are difficult to realize with 2D crystals. Examples range from higher order topological insulators to
perfect Li ion membranes/solid state electrolytes. In my talk I will discuss two examples. First, I will discuss the
synthesis, properties, and applications of atomically thin multilayer amorphous carbon (ML AC), the only
realization of a free standing 2D amorphous material so far [1]. Extensive characterization by transmission
electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated
structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. I
will discuss its potential use as an atomically thin diffusion barrier and as an ultra-low k dielectric. Combined
with its low-temperature synthesis by laser-CVD, these are critical features to enable substantial
improvements in silicon-based semiconductor electronics and ensure compatibility with future 2D
electronics. It also promises to be an exciting material for memristor use.
Next, I will discuss the role of disorder in atomically thin niobium diselenide (NbSe 2 ) intercalated with
dilute cobalt (Co) atoms and show that such systems spontaneously display ferromagnetism below the
superconducting transition temperature (). We elucidate the origin of this phase by constructing a magnetic
tunnel junction that consists of Co and Co doped NbSe 2 as the two ferromagnetic electrodes. At a
temperature well below , the tunnelling magnetoresistance shows a bistable state, suggesting a
ferromagnetic order in superconducting Co-NbSe 2 [2]. We propose a RKKY exchange coupling mechanism
based on spin-triplet superconducting order parameter to mediate such ferromagnetism. Non-local lateral
spin valve measurements with Hanle spin precession signals up to micrometres below suggest an intrinsic
spin-triplet state in superconducting NbSe 2 as key ingredient.

[1] Toh, C.-T. et al., Synthesis and properties of free-standing monolayer amorphous carbon. Nature (2020).
[2] Qu, Tingyu et al., Ferromagnetic Superconductivity in Two-dimensional Niobium Diselenide, under review.