For the first time, an organic and inorganic hybrid 2D material has been synthesized that combines the desirable properties of being electrically conductive and magnetic. The achievement marks the dawn of a new class of materials with possible applications in quantum computing.
Foto: Shutterstock
Denne artikel er fra DTU Kemi Årsrapport 2018. Læs den fulde rapport her.
Since the first synthesis of graphene in 2004, hundreds of 2D materials have been synthesized. However, the novel material chromium-chloride-pyrazine (chemical formula CrCl2(pyrazine)2) is based on a truly groundbreaking concept. While the other known 2D materials are all inorganic – just like graphene – chromium-chloride-pyrazine is an organic/inorganic hybrid material. This allows for highly tunable properties which is advantageous in a range of possible applications, not least in future quantum computers.
An international team led by Assistant Professor Kasper Steen Pedersen, DTU Chemistry, has synthesized the new material.
“This marks a new type of chemistry, in which we are able to replace various building blocks in the material and thereby modify its physical and chemical properties. This cannot be done in graphene. For example, one can’t choose to replace half the carbon atoms in graphene with another kind of atoms. Our approach allows designing properties much more accurately than known in other 2D materials,” says Kasper Steen Pedersen.
A quantum revolution awaits
In principle, a 2D material has a thickness of just a single molecule and this often leads to properties very different from those of the same material in a normal 3D version. Not least the electrical properties will differ. While in a 3D material, electrons are able to take any direction, in a 2D material they will be restricted to moving horizontally through the chemical bonds.
Chromium-chloride-pyrazine is a layered material. Strictly speaking, it is not a 2D material in itself, but a precursor for a 2D material.
Besides the electrical properties, also the magnetic properties in chromium-chloride-pyrazine can be accurately designed. This is especially relevant in relation to the quantum revolution, promising much more powerful computers and improved electronics.
“Almost all other known 2D materials are non-magnetic in their pristine forms, hampering their use in emerging technologies relying on the quantum spin of transported electrons. Examples are spintronics, magneto-electrics, and multi-ferroics. While in normal electronics, only the charge of the electrons is utilized, also their spin – which is a quantum mechanical property – is used in spintronics. This is highly interesting for quantum computing applications. Therefore, development of nano-scale materials which are both conducting and magnetic is most relevant,” Kasper Steen Pedersen notes.
How to tune magnetic properties
Recent years have seen vast efforts in semiconductors doped with transition metals. It is generally believed that such materials will be suited for spintronic applications due to their near-total spin polarization. However, the precise distribution of metalions has so far proven difficult to control, and spatially low-dimensional systems have not been obtained.
As an alternative approach, Kasper Steen Pedersen and his colleagues were inspired by so-called reticular molecule-based metal-organic framework (MOF) chemistry. Here, the synthetic engineering of inorganic and organic modules leads to a range of possibilities for tuning both the physical properties and anisotropy of the chemical bonding in a 3D crystalline solid. Further, reports from other groups had shown promise for the isolation of novel 2D materials. These could either be structured as single sheets or as van der Waals heterostructures. For many 2D materials, the individual sheet is very strong in itself, but when stacked, the binding force between two layers is weak. The sheets are only held together by the van der Waals interaction, which is far weaker than covalent chemical bonds. This might sound like a problem, but in relation to spintronics and similar applications, it is actually an advantage. Since the interactions between the layers are weak, it will only require a small external change – for instance switching a weak magnetic field on and off – to tune the properties back and forth
To introduce strong electronic and magnetic communication between spin carriers in such coordination solids, extensive electronic delocalizationis essential. Indeed, record high electrical conductivities have been obtained in 2D coordination solids of ditopic or polytopic conjugated organic ligands and transition metal ions due to strong π-d conjugation between the ligand and metal ion orbitals.
“However, all of these materials are nonmagnetic. We therefore turned our attention to a possible new type of 2D materials, where both magnetism and electronic conductivity could be tuned.”
Time to investigate stability
Soon, the focus became the simple pyrazine ligand. This common ditopic ligand is found in thousands of crystallographically characterized coordination networks.
Published in the prestigious journal Nature Chemistry, September 2018, the group was able to present the isolation and characterization of a structurally simple layered coordination solid, chromium-chloride-pyrazine, which exhibits both long-range magnetic order and high 2D electronic conductivity.
In addition to quantum computing, chromium-chloride-pyrazine may be of interest in future superconductors, catalysts, batteries, fuel cells, and electronics in general. Still, companies are not ready to produce the material right away, Kasper Steen Pedersen emphasizes:
“This is fundamental research. Since we are suggesting a material synthesized using an entirely novel approach, a number of questions remain unanswered. For instance, we are not yet able to determine the degree of stability of the material in various applications. However, even if chromium-chloride-pyrazine should for some reason prove unfit for the various possible applications, the new principles behind its synthesis will still be relevant.This is the door to a new world of more advanced 2D materials opening up.”
Structure of CrCl2(pyz)2. a) Fragment of the layered structure shown along the Cl–Cr–Cl. b) Perspective view of the staggered stacking of the layers perpendicular to the c direction. c) Thermal ellipsoid plot drawn at 80% probability level showing the positional disorder of the pyrazine rings (dark/light colour). Dark green, Cr; light green, Cl; blue, N; dark grey, C. Hydrogen atoms have been omitted in a and b for clarity.