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Layered functional materials - beyond 'graphene'

Periodic Reporting for period 2 - BEGMAT (Layered functional materials - beyond 'graphene')

Reporting period: 2018-04-01 to 2019-09-30

Since 2015, there are more mobile phone subscriptions than people on Earth, and in each of our electronic hand-held devices, silicon has been processed into thin, semiconducting layers. The importance of electronics – semiconductor manufacturing in particular – poses two challenges world-wide:
(1) In the context of materials security, almost all modern electronic devices depend on small, yet indispensable quantities of critical raw materials (CRMs).
(2) Manufacturing of semiconductors (like Si) requires substantial amounts of energy. Hence, alternative semiconductors that are produced in less energy-intensive ways and that do not require CRMs or complicated post-synthetic modifications for the tuning of desirable properties (e.g. optical/electronic band gap) are a competitive edge for industry and of great interest for academia.

There is an apparent lack of metal-free 2D-matrials for the construction of electronic devices, as only five materials of the “graphene family” are known: graphene, hBN, BCN, fluorographene, and graphene oxide – none of them with a narrow band gap close to commercially used silicon.

This project combines the lessons from kinetic and thermodynamic approaches towards 2D polymers and outlines strategies for design, synthesis, and application of a novel class of materials that extend beyond the carbon chemistry (BEG) of graphene. These BEG-materials combine the unique morphology of the “graphene family” with design principles and diversity from organic chemistry which we see in COFs and conjugated polymers. As such, BEG-materials share the following features:

(1) The BEG-materials are designed in such a way, that a transfer of crystalline order from the molecular (pm-range) to the macroscopic level (cm-range) has been achieved.
(2) Individual, free-standing layers of BEG-materials can be addressed by mechanical or chemical exfoliation.
(3) Assemblies of different BEG-materials (and 2D/3D allotropes) as van der Waals heterostructures with unique properties are possible.
In contrast to the existing “graphene family”,
(4) BEG-materials are constructed in a controlled way by covalent organic chemistry in a bottom-up approach from abundant precursors free of metals and critical raw materials (CRMs).
Moreover – and unlike – many covalent organic frameworks (COFs),
(5) BEG-materials are fully aromatic, donor-acceptor systems. This ensure that electronic properties can be addressed on macroscopic scale in device-like applications such as sensors and switches.
"WP1 – substrate-supported BEG-materials with intrinsic aromaticity. and
We have succsefully synthesised a number of C2, C3 symmetric tectons (based on TTF, DTDAF, triazine, phosphininie), and we obtained a number of 2D layered and 3D amorphous materials as poweders, membranes and on inert substrates (quartz glass) (M1-M5). We can tune the optical, electronic and catalytic properties of these so-called sulphur- and nitrogen-containing porous polymers (SNPs). Importantly, we were able to show that the (opto-)electronic effects in D-A polymers are not simply a function of the size of their π-aromatic domains, but clearly a product of the strength of their intrinsic donor-acceptor interactions. The results were published in three papers:
• “Tailored band gaps in sulphur and nitrogen containing porous donor-acceptor polymers (SNPs)” Bojdys,* M. J. et al. Chem. – Eur. J. 2017. DOI: 10.1002/chem.201703332.
• “Fluorescent sulphur and nitrogen containing porous polymers with tuneable donor‐acceptor domains for light‐driven hydrogen evolution” Bojdys,* M. J. et al. Chem. – Eur. J. 2018. DOI: 10.1002/chem.201802902.
• “Exploring the “Goldilocks Zone” of Semiconducting Polymer Photocatalysts via Donor-Acceptor Interactions” Bojdys,* M. J. et al. Angew. Chem., Int. Ed. 2018. DOI: 10.1002/anie.201809702.

WP2 – catalyst-supported BEG-materials with intrinsic aromaticity.
We obtained a library of heteroatom-doped tectones and converted them info 2D layered (and 3D amorphous) materials on catalytically active 2D interfaces (copper) (M7-M11). This has led to a new paradigm in the construction of 2D/3D van der Waals heterostructures, and in the production of polymer anodes.
• “Twinned Growth of Metal-Free, Triazine-Based Photocatalyst Films as Mixed-Dimensional (2D/3D) van der Waals Heterostructures” Bojdys,* M. J. et al. Adv. Mater. 2017. DOI: 10.1002/adma.201703399
• “Tuning the porosity and photocatalytic performance of triazinebased graphdiyene polymers via polymorphism” Bojdys,* M. J. et al. ChemSusChem 2018, DOI: 10.1002/cssc.201802034
Further, this work has laid the foundation for a patent application and the development of polymer-based high-performance anodes (a project which we aim to develop with our industrial partner VARTA Microelectonics GmbH).
Patent application:
• “Anode und Verfahren zu ihrer Herstellung” (DE 10 2019 110 450.5 Hansen und Heeschen Patentanwälte).

WP3 – substrate-supported BEG-materials with full aromaticity after post-synthetic treatment.
(M16/M17/M19/M20) Using β-amino enone bridges, we show that a 2D layered BEG-material made up of chemoresistant β-amino enone bridges and Lewis-basic triazine (C3N3) moieties has a reversible, real-time ON/OFF response (to HCl and NH3 vapours) that can be followed by the naked eye, in the UV spectrum, and in electrical conductivity measurements.
• “Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework” Bojdys,* M. J. et al. Nat. Commun. 2019. DOI: 10.1038/s41467-019-11264-z
In the context of ""fully aromatic polymers after post-synthetic treatment"" we have investigated incorporation of atoms other than C, N, O, Si and S into conjugated polymer frameworks. Starting with a building block that contains a λ5‐phosphinine (C5P) moiety, we obtain a π‐conjugated, covalent phosphinine‐based framework (CPF‐1); for the first-time we achieved the incorporation of the phosphinine motif into a complex, π-aromatic polymer framework.
• “A π‐conjugated, covalent phosphinine framework” Bojdys,* M. J. et al. Chem. Eur. J. 2019. DOI: 10.1002/chem.201900281

WP4 – processing and modulation of BEG-materials via doping and stacking and WP5 – device-like application of BEG-materials.
We have focused on the model-system of triazine-based graphitic carbon nitride (TGCN) that grows in an epitaxial way on top of insulators (and on itself) (M26) and we on 2D TzF/3D TzG which forms a van der Waals heterostructure based on polymorphism (M25). We were able to show sensor applications (WP5, M30) with these materials, and currently we are working on the incorporation of TGCN into FET-devices.
• “Directional charge transport in layered, two‐dimensional triazine‐based graphitic carbon nitride” Bojdys,* M. J. et al. Angew. Chem., Int. Ed. 2019. DOI: 10.1002/anie.201902314
• “Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework” Bojdys,* M. J. et al. Nat. Commun. 2019. DOI: 10.1038/s41467-019-11264-z

(1) Strong covalent bonds and a “functional” surface area are flaunted as the main advantages of the next generation of covalent organic frameworks [COFs e.g. Yaghi, Dichtel], of conjugated microporous polymers [CMPs e.g. Cooper] and of polymers of intrinsic microporosity [PIMs e.g. McKeown], but – with the exception of recent electrode and catalysis applications – there are few case studies that make genuine use of both these properties. We combine overall π-conjugation, and donor-acceptor domains in predictive, crystalline networks with the sensing application in mind, and we deliver on the promise of “rationally designed, structurally modular frameworks”.

(2) There is an on-going debate in the materials science, and catalysis communities about the question “What makes an ideal heterogeneous photocatalysts for water splitting?”. We consolidate previous attempts at “rational design” and explore the “Goldilocks Zone” for photocatalysis. Highly-modular sulphur and nitrogen containing porous polymers (SNPs) encompass bandgap tunability and efficient charge-transfer that prevents undesired electron-hole recombination. Rational incorporation of D-A diyads into organic photocataysts is only now (in 2019) tackled in earnestness. With three prominent publications in the field, we have contributed to seeding the aspect of D-A interactions in polymer catalyst design.

(3) Only C, N, O, B, Si and S have found their way into rationally designed polymers so far. We expand the toolbox of available building blocks by one additional nonmetal, phosphorous. This is the first time ever that reactive phosphorus found its way into a rationally designed polymer network since a phosphinine-containing molecule was synthesised in 1966.

(4) On-catalysts and on-surface growth from solutions is an entirely underdeveloped concept in materials chemistry, since most groups work with hard-to-control CVD/PVD gas phase deposition techniques. In this respect, our laboratory has pioneered the growth of predictive, 2D crystalline materials on inert substrates and on catalytically active interfaces.