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Bottom-Up Synthesis of Graphene Nanoribbons for Electronic Applications



CMOS transistors are continously scaled to finer geometries and IBM, working with Samsung and GlobalFoundries, demonstrated the world's first 5nm silicon chip in June 2017. This chip was produced by extreme ultraviolet (EUV) lithography, which is an extremely costly and laborious process. CMOS transistor could scale to the 3nm node in the 2021 timeframe and then, CMOS could run out of gas, prompting the need for a new switch technology for beyond CMOS devices.

Graphene nanoribbons (GNRs) with sizes below 3 nm - in contrast to layered 2D graphene - show semiconducting behavior which is derived from lateral electron confinement. Hence GNR tunnel field-effect transistor (TFET) are getting attention as the future generation nanoelectronic devices for the realization of high-frequency low-leakage integrated circuits and so called zigzag edge graphene nanoribbons are discussed for future spintronic applications for example.

State of the art top–down approaches to fabricate GNRs - like lithographic and etching methods, graphene cutting with catalytic particles or unzipping processes of SWCNTs into nanoribbons by chemical agents - are not suitable to produce GNRs with exactly defined electronic properties. Only buttom-up approaches based on chemical reaction of defined precursor molecules allow the production of GNRs with precisely controlled impurity doping, exact energy-level engineering, and defect-free interfaces with well-defined molecular orientations, that are necessary for an improved electronic device performance.


The synthesis of structurally well-defined GNRs has been achieved by extending nanographene synthesis to longitudinally extended polymeric systems.
Access to well-defined GNRs thus becomes possible through the surface-assisted or solution-mediated cyclodehydrogenation of tailor-made polyphenylene precursors.

The "Surface-Bound" Approach

Dihalo-substituted, non-planar oligophenylene precursor monomers are deposited onto suitable conducting metal surfaces (like Au111) leading to GNRs after polymerization and dehydrogenation.
By choosing different dibromo precursors the aspect ratios and
GNR architecture can be varied.

The “Solution” Approach
An efficient AB-type Diels-Alder polymerization for the preparation of polyphenylene precursors with exceptionally large molecular weights was developed. The following intramolecular oxidative cyclodehydrogenation with iron(III) chloride leads to GNRs with lengths up to 200 nm.

The synthetic method developed allows tuning of the width and band gap for liquid-phase-processable GNRs by modifying the monomer architecture, allowing the development of GNR-based optoelectronics, such as photodetectors and photovoltaics.

Offered patent portfolio

MI0903-5280 "Segmented graphene nanoribbons"

EP and US priority applications filed 14.11.2011 - TW201328972A filed 12.11.2012
WO2013/072292 filed 13.11.2012 nationalized in EP, US, JP, KR, CN
CN104039694B, TWI562960B, JP5955398B2, KR101653076B1 granted
EP2780280B1 granted, nationalized in DE, FR, GB
USPTO Notice of Allowance January 2018

nc-AFM image of a short 7-14-7 AGNR quantum dot (scale bar: 2 nm) and schematic energy level diagram of the
7-14-7 AGNR quantum dot. Two red lines indicate a pair of low-energy interface states, and the black lines indicate the levels arising from longitudinal quantum confinement of electrons/holes within the 14-AGNR quantum dot segment.

The above described bottom-up approaches not only enable the fabrication of ultranarrow GNRs, but also give access to GNR heterojunctions with atomic precision.

One option is the production of superlattice structures, in which the width of GNRs is modulated in the longitudinal direction of the GNRs. Band gap engineering between different components of the heterojunction, in a similar manner to superlattices in conventional semiconductors, can then simply be achieved by varying the width of the components rather than their chemical composition, potentially allowing for “all-carbon” device components. The resulting segmented graphene nanoribbons are important for essential electronic components like diodes and tunnel barriers.

MI0903-4884 "Graphene Nanoribbons with controlled modifications"

EP and US priority applications filed 24.05.2012 - TW201402459A filed 22.05.2013
WO2013/175342 filed 13.05.2013 nationalized in EP, US, JP, KR, CN
CN104379497B, TWI580636B, US9676755 granted

Heteroatoms, such as nitrogen, boron or sulfur but also defects can be doped into GNRs. The sites and density of dopants (or defects) are controlled by using a designed precursor. This technology is useable for carrier doping, band tuning, and p–n junction formation.
For example, a process for the fabrication of graphene nanoribbon heterojunctions was developed by combining pristine hydrocarbon precursors with their nitrogen-substituted equivalents. The resulting structures consist of seamlessly assembled segments of pristine (undoped) p-GNRs and deterministically nitrogen-doped N-GNRs, and behave similarly to traditional p–n junctions.
These materials bear a high potential for applications in photovoltaics and electronics.

Band offset across p-N-GNR heterojunctions.
a) Sketch of a heterojunction between a p-GNR segment (left part, grey) and a N-GNR segment (right part, blue). b,d) Computed band structures (along the Γ-X line of the 1D Brillouin zone) of p-GNR (b) and N-GNR (d), both aligned to the Hartree potential in vacuum (that is, energies are given with respect to the vacuum level). c) PDOS of the p-GNR segment (left, grey) and the N-GNR segment (right, blue) of the heterojunction shown in a.

MI4722 "Graphene nanoribbons with controlled zig-zag edge and cove edge configuration"
EP priority application filed 13.02.2014 - TW201538423A filed 11.02.2015
WO2015/121785 filed 09.02.2015 nationalized in EP, US, JP, KR, CN

Another method to design optical and electronic properties of GNRs is the control of the edge structure. For example the edge states of zigzag GNRs are predicted to couple ferromagnetically along the edge and antiferromagnetically between the edges which makes them useable for spintronics. The benefit of spin-tronics is that spin transport, in comparison to electron transport, is more robust against electric fluctuations and more energy-efficient. Alternatively, the level of current and the spin signal can be used simultaneously to improve the processing speed of computers amongst other benefits. As graphene is very rare in its ability to maintain the electron spin over distances needed for spintronics, it is likely to be used in this application. Atomically precise GNR with zigzag edges - with scanning tunnelling spectroscopy proved edge-localized states with large energy splittings - were achieved by bottom-up synthesis through surface assisted polymerization and cyclodehydrogenation of specifically designed precursor monomers.

MI3747 "Triphenylenes for electro luminescent polymers"
EP priority application filed 28.07.2006 - TW200815493A filed 27.07.2007
WO2008/012250 filed 18.07.2007 nationalized in EP, US, CA, JP, CN, KR
US7968872, KR101422055B1, JP5568303B2, CN101495433B granted
EP2046705B1 granted, nationalized in DE, ES, FR, GB, IT, NL

The patent family mainly claims novel polymers comprising triphenylene triangle repeating units (and the corresponding monomers) and their use in polymeric light emitting devices (PLED).

The bromine monomers (3) and the intermediate polymers (4), both claimed as substances by our granted patents, were used by Prof. Alexander Sinitskii and coworkers to produce large band gap graphene nanoribbons according to the following reaction scheme:

T.H. Vo, M. Shekhirev, D.A. Kunkel, M.D. Morton, E. Berglund, L. Kong, P.M. Wilson, P.A. Dowben, A. Enders, A. Sinitskii:
"Large-scale solution synthesis of narrow graphene nanoribbons",

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Bottom-Up Synthesis of Graphene Nanoribbons for Electronic Applications